1. Field of the Invention
The invention relates to a method of communicating in Asynchronous Transfer Mode (hereinafter, referred to as xe2x80x9cATMxe2x80x9d) and, in particular, to a method of communicating by using an AAL (ATM Adaptation Layer) receiving circuit.
2. Description of the Related Art
In the past, a technology about an ATM network has been enthusiastically developed as the Internet has been broadly spread. Information which is transferred in the ATM network is divided into a plurality of ATM cells each of which has 53 bytes of data, and the ATM cells are actually transferred through the ATM network. Each of the ATM cells is constructed of 48 bytes of xe2x80x9cpayloadxe2x80x9d which includes a part of the information and 5 bytes of xe2x80x9cheaderxe2x80x9d which includes a destination address and etc. Furthermore, in the destination address of the header, a VPI (Virtual Path Identifier) and a VCI (Virtual Channel Identifier) are included.
The ATM cells are transferred to the corresponding destination address through a plurality of ATM exchanges located in the ATM network. However, information about a VPI and a VCI is individually defined between a terminal and an ATM exchange, and between ATM exchanges. Therefore, each of the ATM exchanges, to transfer the ATM cells, should convert a VPI and a VCI in the ATM cell to another VPI and VCI that correspond to the next ATM exchange. Such conversion of the VPI and the VCI at the ATM exchanges is referred to as xe2x80x9cheader conversionxe2x80x9d.
A previous technique of the header conversion uses a header conversion table as disclosed in Japanese Laying-Open Publication No. H09-181728 (namely, 181728/1997). In this case, VPIs and VCIs of the outgoing ATM cells are previously stored into the header conversion table in correspondence with VPIs and VCIs of the incoming ATM cells, and the VPIs and the VCIs of the incoming ATM cells are replaced by the corresponding VCIs and VPIs in the header conversion table when the incoming cells arrived.
Therefore, when a header of an ATM cell is converted by using a header conversion table, a large amount of header conversion table is required to store possible ATM cell header information, and a circuit for retrieving the large table and a circuit for conversion are required. As a result, a scale of a circuit for the header conversion on a transmission line interface becomes huge.
Also, the ATM cells may include voice, images, and the other kinds of data. Then, to treat each type of the data, the ATM cells are defined in a plurality of types of protocols (AAL type 1 through type 5). The AAL type of the ATM cells can be identified by referring to a value of a VCI included in the ATM cells.
It often happens that there may be a large amount of vacant area in the payload when an ATM cell is used for sending data of a single user, and when the amount of the data is much less than a payload in the ATM cell.
To avoid such occurrence of the large amount of vacant area, a type of ATM cell has been proposed which can collectively include data which are to be transmitted from a plurality of users and which have different destinations. The type is referred to as AAL type 2 (hereinafter, called as xe2x80x9cAAL2xe2x80x9d) and the type is defined by ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) recommendation I.363.2.
Therefore, an AAL2 cell is normally used when voice information, which has a small size and a short allowable delay time, is transferred. On the other hand, a standard cell is often used on transferring the other information, which has a large size and a long allowable delay time.
In the past, in an AAL receiving circuit, for example, in a radio base station, provision has been made to receive both the standard cell and the AAL2 cell. However, since the earlier AAL receiving circuit must produce an individual header of the standard ATM cell from the AAL2 cell which collects data of a plurality of users, the AAL receiving circuit must include AAL terminating units for the AAL2 cell and AAL terminating units for the standard ATM cell, individually. Further, since it can not be known in advance that either type of the cells is mainly transferred, a large number of both types of the AAL terminating units are needed even if all the units are not fully used. Therefore, the scale of the AAL receiving circuit becomes large, and as a result, the scale of a radio base station including the AAL receiving circuit also becomes large.
In recent years, attention has been focused on a radio communication system employing CDMA (Code Division Multiple Access) as next generation radio communication system. Using a W-CDMA (wide band-CDMA), which is one of the techniques employing CDMA, information between a radio base station and an exchange is transferred in the form of ATM cells. Therefore, in the radio base station including previous AAL receiving circuit as described above, enhancing the number of the communication lines leads to increasing a scale of the circuit, as a result, efficient usage of frequency band, which is advantage of CDMA, is degraded.
More detail description is made about the previous AAL receiving circuit.
Eight bits of a field are assigned to a VPI based on the UNI (User Node Interface), and 12 bits of field are assigned based on the NNI (Network Node Interface). A field of sixteen (16) bits is assigned to a VCI based on UNI and NNI. FIG. 1 shows a header format of an ATM cell based on UNI.
The header also includes three bits of PT (Payload Type), a single (8) bit of CLP (Cell Loss Priority), and an eight bits of HEC (Header Error Control).
Next, description is made about the case where messages of users A, B, and C shown in FIG. 2A will be sent through the same path until they reach an intermediate portion located on the way to their destinations. Here, the message of the user A has twelve (12) bytes length, the message of the user B has eighteen (18) bytes length, and the message of the user C has six (6) bytes length. When these messages are sent through an ATM cell other than AAL2 type cell (hereinafter, referred to as xe2x80x9cstandard cellxe2x80x9d), each of the messages is included in a ATM cell different from each other as shown in FIG. 2(b). In FIG. 2B, an ATM cell containing the message of the user A has 36 bytes of vacant space out of 48 bytes payload. The vacant space is shown as a shaded portion. Similarly, an ATM cell containing the message of the user B has a 30 bytes of vacant space, and an ATM cell containing the message of the user C has a 42 bytes of vacant space.
FIG. 2C shows format of an ATM cell when the above messages are sent in AAL2 cell.
As shown in FIG. 2C, the AAL2 cell includes short cells (also called xe2x80x9cCPS (Common Part Sub-layer) packetxe2x80x9d) containing the messages of users A, B. and C, and STF (start field) 111. Each of the short cells includes the message of the user (short cell payload) and the short cell header (110a, 110b, or 110c). The STF 111 has 1 byte length and each of the short cells (110a, 110b, and 110c) has 3 bytes length. Therefore, the 48 bytes of payload of the ATM cell shown in FIG. 2C is occupied by 46 bytes of data (1 (STF 111) +3 (short cell header)xc3x973+12+18+6=46), and the only 2 bytes of area is unused.
Here, more detail description is made about the above AAL2 cell with reference to FIG. 3.
As described above, the 48 bytes of payload in the AAL2 cell includes the 1 byte of STF 111 and a plurality of short cells.
The STF 111 further includes 6 bits of OSF (offset field), a 1 bit of SN (sequence number), and a 1 bit of P (odd parity of the STF). In FIG. 3, each of the numbers in parentheses indicates the number of bits of the corresponding data. The OSF indicates a boundary of a first short cell.
Also, each of the short cells includes 3 bytes of short cell header and a short cell payload which has a variable length. The short cell header includes 8 bits of CID (Channel Identifier), 6 bits of LI (Length Indicator), 5 bits of UUI (User-to-User Indication), and 5 bits of HEC (Header Error Control). The CID includes destination information to which the short cell is to be transferred in a manner similar to a VCI.
Next, description is made about an AAL receiving circuit which sends/receives ATM cells including different AAL types.
FIG. 4 shows a block diagram of a radio communication system in which a radio base station which includes an AAL receiving circuit of the previous technique.
The radio communication system includes a plurality of mobile stations 201 and 202, a radio base station 88, and an exchange 1 which is connected to the radio base station 88 via an ATM transmission line 10.
Also, the radio base station 88 includes radio units 831 to 83n, n in number, radio units 931 to 93n, n in number, and an AAL receiving circuit 82.
In FIG. 4, the mobile station 201 communicates voice information via a radio network between the mobile station 201 and the radio unit 93n. Further, the mobile station 202 communicates data via a radio network between the mobile station 202 and the radio unit 831, and transfers the data to a personal computer 22 via an adapter 21.
The AAL receiving circuit 82 includes an AAL identifying unit 3, a standard cell interchanging unit 4, an AAL2 assembling unit 5, a header converting unit 81, AAL terminating units, n in number, for standard cell 871 to 87n, and AAL terminating units, n in number, for AAL2 cell 971 to 97n. 
The AAL identifying unit 3 determines a value of a VCI in an ATM cell from the ATM transmission line 10 and identifies the type of the ATM cell based on the value of the VCI. When the ATM cell is identified as an AAL2 cell, the ATM cell is transferred to the AAL2 assembling unit 5. On the other hand, when the ATM cell is identified as a standard cell and judged as a cell other than an AAL2 cell, the ATM cell is transferred to the standard cell interchanging unit 4.
The standard cell interchanging unit 4 sends the received standard cell to the AAL terminating units for standard cell 871 to 87n without any change.
The AAL2 assembling unit 5 disassembles the received AAL2 cell into a plurality of cells each of which corresponds to one of short cells and appends an original ATM header to each of the cells to produce a standard cell.
Also, the AAL2 assembling unit 5 includes, as shown in FIG. 5, an OSF terminating unit 51, a short cell assembling unit 52, an ATM header appending unit 53, and a standard cell assembling unit 54.
The OSF terminating unit 51 terminates an offset field included in a payload of the received AAL2 cell.
The short cell assembling unit 52 assembles each of the short cells included in the received AAL2 cell as individual cells.
The ATM cell header appending unit 53 appends a header to each of the individual cells. Then, the standard cell assembling unit 54 assembles each of the cells into a cell having the same format as the standard cell.
Next, returning back to FIG. 4, the header converting unit 81 converts a header of each of the cells assembled by the AAL2 assembling unit 5 into a header including the corresponding destination of the user message. In particular, the unit 81 sets each of the CIDs in short cells into the corresponding header, which is appended by the ATM cell header appending unit 53. The above conversion by the header converting unit 81 is achieved using the header conversion table which includes new headers each of which is related to a header to be replaced by it.
Then, detailed description is made about processing executed by the AAL2 assembling unit 5 and the header converting unit 81 with reference to FIG. 6.
Format of the AAL2 cell sent from the AAL identifying unit 3 is shown in FIG. 6A. The AAL2 cell includes a header 200 and a payload. The STF in the payload includes the OSF 203. Further, the payload includes one short cell including a short cell header 204a and a short cell payload 205a, and the other short cell including a short cell header 204b and a short cell payload 205b. 
The AAL2 cell shown in FIG. 6A is converted into two ATM cells shown in FIG. 6B by processing of the AAL2 assembling unit 5. Headers 200 of the two ATM cells are similar to the ATM cell header 200 shown in FIG. 6A. In the illustrated example, a payload in one ATM cell includes a short cell header 204a and a short cell payload 205a, while a payload in the other ATM cell includes a short cell header 204b and a short cell payload 205b. 
Then, the two ATM cells are each converted to produce two individual ATM cells shown in FIG. 6C by performing header conversion at the header converting unit 81. Each of payloads in the two ATM cells shown in FIG. 6C includes only a short cell payload (205a or 205b).
Next, more detail description is made about header conversion of the header converting unit 81 with reference to FIG. 7. In this case, header conversion of an ATM cell is performed. The ATM cell includes a short cell including a short cell header 204a and a short cell payload 205a (not shown). In this example, a CID is set into higher significant eight bits of a VCI, when all of the higher significant eight bits of the VCI have been originally set to zero and unused.
FIG. 7A shows a format of the ATM cell before header conversion is performed, while, FIG. 7B shows format of the ATM cell after header conversion is performed.
As shown in FIG. 7A, the header 200 of the ATM cell before header conversion includes 8 bits of VPI and 16 bits of VCI, and a short cell header 204a including 8 bits of CID.
Then, the header converting unit 81 converts the header 200 by replacing the 16 bits of VCI with the 8 bits of CID at the high order 8 bits, and the low order 8 bits of the original VCI remain in the low order 8 bits. By the conversion, the header 200 is converted to the header 201.
The two ATM cell headers 200 shown in FIG. 6B are identical to each other, while destinations are different from each other. Therefore, the two ATM cell headers must be converted so that each of the ATM cells is transferred to correct destination.
However, as described above, when a header of an ATM cell from the AAL2 assembling unit 5 is converted by the header converting unit 81, the converted ATM cell does not have a standard format. Therefore, the converted ATM cell can not be treated as similar to a standard ATM cell which passes through the standard cell interchange unit 4.
As a result, AAL terminating units for a standard cell 871 to 87n and AAL terminating units for an AAL2 cell 971 to 97n must be separately located.
Each of the AAL terminating units for a standard cell 871 to 87n detects a destination included in a standard ATM cell sent from the standard cell interchange 4, and transfers the information included in a payload of the ATM cell to one of the radio units 831 to 83n if the destination is a predetermined one.
Each of the AAL terminating units for an AAL2 cell 971 to 97n detects a destination included in an ATM cell sent from the header converting unit 81, and transfers the information included in a payload of the ATM cell to one of the radio units 931 to 93n if the destination is a predetermined one.
Then, each of the radio units 831 to 83n and 931 to 93n sends the information from the AAL terminating units for a standard cell 871 to 87n and the AAL terminating units for an AAL2 cell 971 to 97n, to each mobile stations 201 and 202 via a radio network.
The previous AAL receiving circuit 82, for example, in a radio base station, converts a header of the AAL2 cell into a header including a CID rather than a standard format header. Therefore, the AAL receiving circuit 82 must include an AAL terminating units for the AAL2 cell and an AAL terminating units for the standard cell, separately.
However, since the number of cells which are received is not known for each AAL type of cell when the radio base station has n of the communication lines, n of the AAL terminating units for a standard cell and n of the AAL terminating units for an AAL2 cell are needed even if all the units are not actually used. Therefore, a scale of the AAL receiving circuit 82 becomes large, as a result, a scale of the radio base station including the AAL receiving circuit also becomes large.
Therefore, it is an object of the invention to provide an improved AAL receiving circuit operable to commonly treat AAL2 cells and standard cells in a single AAL terminating unit.
Further, it is an another object of the invention to provide an improved AAL receiving circuit which can reduce a scale of circuit by eliminating header conversion.
According to a first aspect of the invention, there is provided an AAL receiving circuit which includes an AAL identifying unit which identifies an AAL type of the ATM cell based on a value of a VCI included in the ATM cell, a standard cell interchanging unit which receives the ATM cell and transfers the ATM cell with no change when the ATM cell is determined as a standard cell at the AAL identifying unit, an AAL2 assembling unit which receives the ATM cell, divides the ATM cell for each short cell, appends a header of the ATM cell to each of the divided ATM cells, converts them in a form of the standard cell, and transfers them as an ATM cell, when the ATM cell is determined as an AAL2 cell, a cell multiplexer which multiplexes the ATM cells which are transferred from the standard cell interchanging unit and the AAL2 assembling unit, and transfers them to a common ATM bus, and at least one AAL terminating unit which determines the AAL type of the ATM cell which is transferred from the common ATM bus and processes the ATM cell according to the determined AAL type.
According to a second aspect of the invention, there is provided an AAL receiving circuit which includes an AAL identifying unit which identifies an AAL type of the ATM cell based on a value of a VCI included in the ATM cell, a standard cell interchanging unit which receives the ATM cell and transfers the ATM cell with no change, when the ATM cell is determined as a standard cell at the AAL identifying unit, an AAL2 assembling unit which receives the ATM cell, divides the ATM cell for each short cell, appends a header of the ATM cell to each of the divided ATM cells, converts them in a form of the standard cell, and transfers them as an ATM cell when the ATM cell is determined as an AAL2 cell, a cell multiplexer which multiplexes the ATM cells which are transferred from the standard cell interchanging unit and the AAL2 assembling unit, and which transfers them to a common ATM bus, a cell distributing unit which determines the AAL type of the ATM cell which is transferred from the cell multiplexer and transfers the ATM cell with processing according to the determined AAL type, and at least one AAL terminating unit which processes the ATM cell from the cell distributing unit according to the determined AAL type.
According to a third aspect of the invention, there is provided a method of AAL processing of an ATM cell. Wherein, the method comprises the steps of identifying an AAL type of a received ATM cell based on a value of a VCI of the ATM cell, dividing the ATM cell for each short cells, appending a header of the ATM cell to the short cells, and converting them into the same form as a standard ATM cell, only when the ATM cell is determined as an AAL2 cell, multiplexing a standard ATM cell and the ATM cells which are obtained by the above conversion, and transferring them to a common ATM bus, and referring to an AAL type of the ATM cell from the common ATM bus, processing the ATM cell according to predetermined definition which corresponds to the standard cell when the ATM cell is determined as a standard cell, determining a destination of the ATM cell by referring to a CID in a header of the ATM cell and processing the ATM cell according to predetermined definition which corresponds to the AAL2 cell when the ATM cell is determined as a AAL2 cell.
According to a fourth aspect of the invention, there is provided a method of AAL processing of an ATM cell. The method comprises the steps of identifying an AAL type of a received ATM cell based on a value of a VCI of the ATM cell, dividing the ATM cell for each short cells cell, appending a header of the ATM cell to the short cells, and converting them into the same form as a standard ATM cell only when the ATM cell is determined as an AAL2 cell, determining a destination of the ATM cell from a VPI/VCI in a header of the ATM cell and transferring the ATM cell to a communication network which corresponds to the destination when the ATM cell is determined as a standard cell, and determining a destination of the ATM cell by referring to a CID in a header of the ATM cell and transferring the ATM cell to a communication network which corresponds to the destination when the ATM cell is determined as an AAL2 cell, and processing, at the communication network, the ATM cell according to the AAL type of the ATM cell.
According to a fifth aspect of the invention, there is provided a receiving circuit which includes a determining unit which determines whether the received packet has a first type defined by a standard structure or a second type defined by a structure including multiple messages each of which is to be sent in a packet, a standard packet interchanging unit which receives the packet and transfers the packet with no change when the packet is determined as a packet of the first type at the determining unit, an assembling unit which is operable in response to the second type of the received packet and which receives the packet, divides the packet for each messages in the packet, appends a header of the packet to each of the divided messages, converts them in a form of an individual packet, and transfers them as a packet, a packet multiplexer which multiplexes the packets transferred from the standard packet interchanging unit and the assembling unit, and which transfers them to a common bus, and at least one terminating unit which determines either one of the first and the second types transferred from the common bus and which processes the packet according to the determined type of the packet.
According to a sixth aspect of the invention, there is provided a method of receiving a packet. The method comprises the steps of (a) determining whether the received packet has a first type defined by a standard structure or a second type defined by a structure including multiple messages each of which is to be sent in a packet, (b) receiving the packet and transferring the packet with no change when the packet is determined as a packet of the first type, (c) receiving the packet, dividing the packet for each messages in the packet, appending a header of the packet to each of the divided messages, converting them in a form of an individual packet, and transferring them as a packet, when the packet is determined a packet of the second type, (d) multiplexing the packets which are formed by (b) and (c), and transferring them, and (e) determining either one of the first and the second types and processing the packet according to the determined type.